The driving whip should be able to reach the shoulder of the farthest horse (the
only horse in a single hitch, or the Leaders in a Four-in-Hand or a Tandem, etc.). I like a fairly long lash, because I use it to cue the horse. I touch him on the belly for forward movement and on the back and/or sides for lateral movement. A long lash helps me cue the horse without affecting my rein in my hand because I can "flick" the whip. I hardly ever use the shaft (hard part) of the whip to cue the horse except for maybe very green horses that don't understand what I am asking for with the lash. Eventually, we only use the lash. Using the shaft is very "novice".
Now, that being said, that is how you use a whip for an ADS-style turnout. Breed show turnouts tend to like shorter whips, but a lot of those drivers also don't really "use" the whip. Their rules are also a bit different in that the whip should not reach
past the girth. That would be too short for ADS.
My favorite whip is a 48" Westfield whip with a 24" lash. It was about $20. It is too short to show in, so I have some carbon fiber whips from PHSsaddlery.com for show. Both are pretty well-balanced. A lot of people like Ultralite Whips, but I find they are
too light for my taste. Many people with carpal tunnel and the like are fond of Ultralites.
Remember though, you shouldn't have to
hold a whip. It should balance on your thumb and push up on the "butt" of your hand, at about a 45 degree angle. Holding it this way will REALLY lessen how tired your hand/arm gets. And practice will also make it easier. I pretty much feel "naked" trying to drive without one now!
Myrna